During cell division, cells move identical copies of chromosomes into two daughter cells. Incorrect segregation of chromosomes is not tolerated by cells, and can lead to cell death, as well as birth defects and cancer. To segregate chromosomes, the cell constructs the mitotic spindle, a football-shaped structure comprised of large protein filaments called microtubules, which are made up of tubulin subunits. Microtubules are "dynamic", meaning they go through periods of growing and shrinking, gaining and losing subunits from their ends. Microtubule ends attach to large protein complexes on the duplicated chromosomes called kinetochores, and this attachment allows movement of chromosomes along with microtubule growing and shrinking. In order for kinetochore attachment to be able to translate microtubule dynamics into chromosome movement, the kinetochore must be attached "end-on", in such a way that the very end of the microtubule is buried in the kinetochore structure. An end-on attachment must be strong enough to move an entire chromosome, but dynamic enough that the attachment can be retained even as thousands of tubulin subunits are added and lost from the site of attachment. While we understand which protein components of the kinetochore are required to bind to microtubules, how end-on attachment is achieved is still mysterious. This proposal outlines experiments using single molecule assays to determine the mechanism and protein requirements for end-on microtubule binding to kinetochores. PUBLIC HEALTH RELEVANCE: The research we have proposed has high relevance to cancer progression and tumorigenesis, as many cancers posses chromosome segregation defects, and many kinetochore proteins are overexpressed in cancer cells. Understanding how segregation occurs normally with help us to understand what goes wrong in cancer cells, and will reveal more targets for cancer therapeutics.